334 
Fishery Bulletin 1 10(3) 
sible to cover all transects on each successive pass. The 
core area, or the common area covered on all passes 
(Fig. 1), was used in further analyses to ensure that 
similar areas were used in comparisons between passes 
made on different days and between pairs of passes. 
Acoustic equipment and backscatter processing 
Acoustic measurements were collected with a calibrated 
Simrad 3 (Kongsberg AS, Horten, Norway) EK60 sci- 
entific echo sounding system (Simrad, 2004) with 5 
split-beam transducers (18, 38, 70, 120, and 200 kHz) 
and a Simrad ME70 multibeam echo sounder (Trenkel 
et ah, 2008). The split-beam transducers were mounted 
on the bottom of a retractable centerboard, positioning 
the transducers 9.15 m below the water surface during 
survey activities. A pulse length of 0.512 ms and ping 
rate of 1.0 s were used for all EK60 data collections. 
Nominal half-power beam widths were 7° for the 38-, 70-, 
120-, and 200-kHz transducers and 11° for the 18-kHz 
transducer. Acoustic instruments on the Oscar Dyson, 
other than the split-beam and multibeam systems, were 
turned off (e.g., the navigational fathometer, Doppler 
speed log) during acoustic data collections. Data process- 
ing and analyses of the acoustic data were performed 
with Echoview software, vers. 4.70.48 (Myriax Software, 
Hobart, Tasmania, Australia). The 38-kHz echo sounder 
was the primary source for the quantitative rockfish 
backscatter measurements presented here. To mea- 
sure performance of the EK60 system, acoustic system 
calibrations with a standard target were conducted by 
following the methods of Foote et al. (1987). 
The echo sounders estimated the distance 
to the bottom with the amplitude-based algo- 
rithm (with a threshold of -36 dB re 1 m -1 ) 
implemented in the echo sounder software 
(Simrad ER60, vers. 2.1.2). The mean of the 
sounder-detected bottom from all 5 frequen- 
cies of the EK60 echo sounder was used as 
the bottom discrimination line in further 
data processing (Jones et al., 2011). Acoustic 
measurements were integrated from 16 m be- 
low the surface to the bottom discrimination 
line. All echograms were examined for bot- 
tom integrations. Acoustic backscatter was 
averaged at 2 resolutions: 185 m (0.1 nmi) 
horizontal by 1) 0.5 m vertical down to 0.5 
m above the bottom discrimination line and 
2) 0.25 m vertical from 0.5 m to the bottom 
discrimination line. All data were exported 
using an S v integration threshold of -70 dB 
re 1 m -1 . 
Based on calculations from Ona and Mit- 
son (1996), the near-bottom acoustic dead 
zone calculated with the current system 
configuration was about 0.3 m at a depth of 
100 m. With an additional zone of partial 
integration (where part of the sampled vol- 
ume is in the dead zone) equivalent to ~0.2 
m and a backstep of 0.25 m (to ensure that 
backscatter from the seafloor is excluded), 
the total integrator dead zone at a depth 
of 100 m was -0.7 m above the sounder- 
detected bottom. 
Backscatter was designated to a catego- 
ry (i.e., rockfishes on the bank, deep rock- 
fishes, bubbles, or zooplankton mix) based 
on backscatter morphology, location on the 
bank, depth in the water column, and fre- 
quency response. Backscatter attributed 
to rockfishes was assigned to 2 categories 
based on location in the water column and 
whether the rockfishes were located on the 
shallow bank or deeper adjacent shelf break 
(i.e., bank flanks). Thus, backscatter in one 
category, hereafter referred to as rockfishes 
3 Mention of trade names or commercial 
companies is for identification purposes 
only and does not imply endorsement by 
the National Marine Fisheries Service, 
NOAA. 
Kodiak Island 
■eK--. 
njlJ l 1 I Kilometers 
0 1 2 4 6 8 
SHffl Rockfish on bank 
■H Deep rockfish 
Sampling tool 
® SDC=9 
☆ ROV=5 
O Trawl=6 
Region 
Core area 
f : Marginal trawlability 
I ' 1 Untrawlable 
I I Trawlable 
Figure 1 
Location of the survey site on Snakehead Bank in the Gulf of Alaska near 
Kodiak Island, Alaska. Parallel lines represent the full extent of tran- 
sects surveyed and the core area is represented by the rectangle outlined 
in black in the middle of this bank. Other colored polygons represent 
trawlability, which was determined with multibeam acoustic backscat- 
ter. Symbols indicate sites where the stereo-video drop camera (SDC), 
remotely operated vehicle (ROV), or bottom trawls were deployed. Green 
bars depict acoustic backscatter (s A m 2 nmi -2 ) attributed to rockfishes 
(species mix) on the bank. Red bars depict rockfishes (e.g., Pacific ocean 
perch [Seftastes alutus ]) detected at depths >150 m along the bank flanks. 
The height of the scale bar for acoustic backscatter represents 19,000 s A . 
